Methods for sea water de-salination have been proposed in a number of different technologies. Two major methods are described here as examples:
(a) Distillation Method
One form of this method involves raising the temperature of sea water at atmospheric pressure above 100° C. to produce steam. The steam is then distilled using either ambient air or water as the cooling medium to condense the steam back to pure water. This method has the following advantages:    1. Most bacteria are killed off as the water temperatures are above 65° C.;    2. Systems are easily monitored due to visible boiling taking place;    3. Principal of operation is easy to comprehend so little personnel training is needed;    4. Total Dissolved Solids (TDS) of <10 ppm are left in the distilled water. However this method also has a number of disadvantages including:    1. Very inefficient as indirect methods to heat the sea water are typically used;    2. At high temperatures any solids in the sea water will congeal. The congealed solids are deposited on the surface of the heat exchanger causing further reduction in efficiency.
Another form of this method of distilling sea water is to reduce the temperature of the boiling point of water by lowering the pressure in the distillation chamber. This method is favored on ships due to its lower energy consumption. The major drawback with this system is that the distillation temperature is too low to kill bacteria and ultra violet lamp generated ozone is usually needed to kill bacteria in the distilled water. It is also not suitable for large scale applications due to the difficulty of maintaining the distillation chamber at below atmospheric pressures.
(b) Reverse Osmosis
Reverse osmosis (R.O.) is a filtration system that uses a membrane to remove ionic, organic and suspended solids from a water supply. Unlike conventional filtration, a membrane system separates the feed water into two streams, a permeate stream and a concentrate stream. The permeate stream is the water that passes through the semi-permeable membrane, while the concentrate stream is the part of the feed stream that is used to flush the concentrated solids from the system.
A pump is used to feed water to membrane housings of the membrane system. The direction of the water flow is indicated by an arrow on individual housing. Water is separated by the membrane within the housing and leaves the membrane housing in two streams as permeate and concentrate.
The permeate is collected at the permeate manifold for systems where more than one membrane housing is used. One permeate line per membrane housing is useful. A pressure relief valve or a pressure switch is usually installed to protect the system against permeate stream over pressurization. The permeate then flows through a flow meter to the outlet point of the machine.
The concentrate leaves the last membrane housing and is split into two streams, the concentrate and the recycle, each with its own adjustable flow control valve. The concentrate valve has three functions namely: controlling the pressure within the machine, controlling the amount of concentrate flowing to the drain, and assisting in controlling the system recovery. A recycle valve channels a predetermined amount of concentrate into the pump inlet to achieve more turbulent cross flow.
The membrane will perform differently at varying pressures and will reject mono and polyvalent ions at a different rate or effectiveness.
This method relies very much on the efficiency of the pumps that drive the R.O. system and by their nature pumps are very inefficient users of energy.